The present invention relates to a method of and a device for controlling a serial printer, such as a dot matrix printer, and particularly to a method of controlling the spacing operation and line-feed operation when the normal spacing is obstructed.
Dot matrix printers have a print head and an ink ribbon cassette mounted on a carriage. The print head has print wires that are selectively driven toward a print medium on a platen. Printing is achieved by selectively driving the print wires while moving the carriage laterally across the print medium. This lateral movement is called spacing. The spacing is conducted in the forward direction (from the left to the right, as seen from the front of the platen) or in both forward and reverse directions. After printing of each line is completed, the print medium is moved (by moving the platen, for example) longitudinally by one line. This longitudinal movement is called line-feed. A sequence consisting of printing while spacing and then line-feed is repeated to perform printing over all the surface of the print medium. In such a printer the tip of the print head is disposed close the the surface of the print medium. It is therefore possible that the print head may be stopped by abutment with a projection of a print medium.
This will be described in further detail with reference to FIG. 4, which is a partial perspective view of a carriage in a conventional dot matrix printer.
As illustrated, a print head 1 is mounted on a carriage frame 2, and is disposed such that it is capable of sliding, together with the carriage, to the right and to the left on a carriage shaft 3. A space motor 4 is mounted on the carriage. The stator of the space motor 4 is also the carriage frame 2.
When the space motor 4 rotates, the pinion 5 rotates to move the space rack 6 back and forth, with the result being that the carriage on which the space motor 4 is mounted is moved to the right and to the left.
Provided on the carriage is a slit disk 7 which moves together with the space motor 4. A pulse generator 8 comprises a light-emitting element such as a light-emitting diode, and a photosensor disposed to receive light from the light-emitting element, and produces pulses at a rate proportional to the rotational speed of the space motor 4.
In the above dot matrix printer, the print head 1 is moved in the forward and reverse directions (this operation is the spacing operation), and at timings in synchronism with the spacing operation, the print wires of the print head 1 are struck against the print medium on the platen, so that printing is effected.
When printing of each line ends, the print medium on the platen is line-fed by means of the line-feed motor, and the print head 1 is moved to the position at which the printing of the next line begins, and printing is again started.
During printing, when the load on the carriage is temporarily increased, or when the torque of the space motor 4 is temporarily decreased, the carriage may stop, causing an error.
In such a case, even when the operator tries to restart the printer, the spacing operation will not be resumed unless the cause of the stoppage of the carriage is removed.
A solution in the prior art is to increase the torque of a space motor 4, making the occurrence of the carriage stoppage more difficult. However, this was undesirable because of the resultant increase in the size and the cost of the space motor 4 and the motor drive circuit. Recently, a control device has been proposed in which the carriage control is achieved without increasing the torque of the space motor 4.
In this control device, when the carriage is stopped, it is moved backward to the original position, i.e., where the spacing operation is started, and then the carriage is again moved forward. If the carriage is stopped again at the same position, recognition is made that the error is not recoverable and an error indication is made. If the carriage is not stopped, the spacing operation is continued and the printing is resumed.
It is, however, often the case that the cause of the stoppage of the carriage is not removed by such an operation, and rather the situation becomes worse by such an operation. This is true where the cause of the stoppage is abutment of the print head with a projection of multiple sheet paper (consisting of several sheets laminated with each other). An example of such a multiple sheet paper is shown in FIG. 5A and FIG. 5B, which are a cross sectional view and a perspective view, respectively. The multiple sheet paper 31 having feed perforations 32 is provided with a projection 33 in the form of an embossment or piercing provided to hold the sheets together. In such a case, sheets of the multiply paper may be turned over by abutment with the print head, and the projection becomes higher such that the projection is not eliminated but rather becomes worse.